1. Field of the Invention
The present invention relates to a fixing device mounted in an electrophotographic image forming apparatus, such as a laser printer or a laser facsimile, and an image forming apparatus including the same.
2. Description of the Related Art
In an electrophotographic image forming apparatus, a photosensitive drum is uniformly charged and exposed by a laser scanning unit to form an electrostatic latent image thereon according to an image signal. A charged toner is supplied to the photosensitive drum to develop the electrostatic latent image with a developer and form a toner image. The toner image is transferred onto a sheet (a recording medium). Since the toner image transferred onto the sheet is not fixed, the toner image is heated and pressed by a fixing device included in the image forming apparatus such that the toner image is fused by heat to form an image fixed on the sheet.
A conventional fixing device generally includes a cylindrical metal heat roller (an electric heating member), a press roller (a pressurizing member) contacting and applying pressure to the heat roller, and a halogen lamp (a heat source) that is mounted in the heat roller and heats the heat roller by radiant heat. In the fixing device (a two-roller type), a sheet having a toner image is inserted and carried into a fixing nip unit formed by the heat roller and the press roller. The toner image is then heated by the heat of the heat roller and fixed on the sheet by the pressurized contact between the heat roller and the press roller.
In the conventional fixing device, it takes a long time to heat the heat roller from a room temperature to a fixing temperature of 160 to 200° C. Accordingly, warming-up time from the time of applying power to the time of starting a printing operation is significantly long. Furthermore, to shorten the period from a print standby time to a print start time, the fixing device should be maintained at a high temperature. However, to maintain the fixing device at a high temperature, the fixing device should be heated by a halogen lamp, thus increasing power consumption.
Accordingly, a fixing device that can shorten the warning-up time and reduce power consumption has been suggested.
For example, an electromagnetic induction heat type fixing device includes a cylindrical or semi-circular holder. A cylindrical film is formed on the holder by laminating a conductive layer and a resistance layer. A magnetic field generating means is composed of an excitation coil and a magnetic core and mounted in the holder. A press roller presses and contacts the outer circumferential surface of the film to form a fixing nip portion. According to the electromagnetic induction heat type fixing device, an eddy current is generated by the magnetic flux generated by the magnetic field generating means. Joule's heat is generated in the conductive layer by the eddy current, and the temperature of the fixing nip portion increases by the Joule's heat. When the fixing nip portion is heated to a fixing temperature by the electromagnetic induction heat type fixing device, the temperature of the fixing nip portion can increase to the fixing temperature in a short time and the warming-up time can be shortened. Furthermore, since the film has a low heat capacity, the power consumption can be reduced. This type of fixing device is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2004-126274. However, since this fixing device needs the magnetic field generating means, the component cost increases compared with the two-roller type fixing device. Additionally, since the inner circumferential surface of the film slides on the holder, the film wears fast and thus the life span of the film is reduced. Additionally, the range of material selection is narrow.
Furthermore, a press belt pressing type fixing device has a tube type press belt instead of the press roller included in the two-roller type fixing device and presses a portion of the press belt at the inside thereof toward a heat roller to form a fixing nip portion. This press belt pressing type fixing device includes a heat roller that is heated by a heat source, such as a halogen lamp, and axially rotates. A caterpillar press belt contacts and follows the heat roller. A pressurizing member presses the surface of the press belt at the inside thereof toward the heat roller. This press belt pressing type fixing device is disclosed in Japanese Unexamined Patent Application Publication No. 2004-12682. According to this fixing apparatus, since the surface-pressed press belt contacts the heat roller with pressure, a wide fixing nip portion is formed. Thus, the warming-up time can be shortened, the power consumption can be reduced, and the component cost can be reduced compared to the electromagnetic induction heat type fixing device. However, since the inner circumferential surface of the press belt is surface-pressed and slides, the press belt is easily worn. Thus, the life span of the press belt or the pressurizing member is reduced. Additionally, the range of material selection is narrow.
Furthermore, a heat belt type fixing device employs a heat belt unit using a heat belt, instead of the heat roller included in the two-roller type fixing device, and contacts the heat roller with a press roller with pressure to form a fixing nip portion. The heat belt unit is composed by winding a caterpillar heat belt on a support roller and a cylindrical heat roller. A cylindrical press roller contacts an interposing portion of the heat belt installed between the support roller and the heat roller. A halogen lamp is mounted in the heat roller or the press roller. According to the heat belt type fixing device, since the wide fixing nip portion is formed, the warming-up time can be shortened and the power consumption can be reduced. Furthermore, since there is no sliding portion, no member is worn and various materials can be used. Additionally, the component cost can be reduced more compared to the electromagnetic induction heat type fixing device. This fixing device is disclosed in Japanese Unexamined Patent Application Publication No. 2004-205877.
However, in the conventional heat belt type fixing device, since the heat belt expands by the support roller and the heat roller and is interposed in a tensed state, if the heat roller rotates, the heat belt moves in an axial direction of the heat roller or the support roller and deviates from a predetermined trajectory due to manufacturing errors or assembling errors of each roller and fluctuation of the heat belt in a width direction.
Accordingly, a need exists for an image forming apparatus having an improved fixing device that surely fixes a toner image onto a recording medium while reducing warm-up time, power consumption and belt deviation.